I kind of expected Fl to react to photos indifferently and continue on with her affair as if nothing happened. I don't really know why she reacted the way she did, but i guess it was because she is scared the image of the normal family and all the expectations her parents and others put on it will crumble. She married just to conform and now she is faced with the destruction of her socially comfortable life.
So, as far as my puny understanding can reach: Jiwon likes Myeongin romantically, but hasn't yet realized her feelings or/and unconsciously suppresses them. At first, she wasn't thinking about kisses in a "friendly" way. After the first kiss, which has started the whole charade, she believed it was something more. Then her train of thought took the wrong turn and she assumed that Myeongin was likely just playing with her. So she started doing the same perceiving her desires as lust. Jiwon openly speaking about previous boyfriends with Myeongin may have something with her being jealous because She thinks Myeongin has kissed others before.
So they both are thinking that other one doesn't have romantic feelings for them. Myeongin just accepted it silently, and Jiwon, while not admitting her feelings, actually tries to monopolize Myeongin. What a mess.
Actually, he is not a coward. Pretty sure, standing against the dragon requires quite a lot of courage(or stupidity). Though he is afraid, he still does whatever he has to do. After all, the only time a man can be brave is when he is afraid.